In the past, starting of large aircraft jet engines has normally been accomplished with pneumatic starters, which derive their air energy from external power; for example from an auxiliary power unit (APU) or bleed air from another jet engine. With the advent of the all electric airplane, presently under consideration by the aircraft industry, ducting for subsystems such as the environmental control system is to be eliminated in the interest of saving weight. Moreover, one of the primary objectives of the all electric airplane is the elimination of engine bleed which of course eliminates cross-bleed engine starts. Thus the future development of advanced all electric type aircraft necessitates the development of efficient electric starting technology. Such a system not only would eliminate ducting and engine bleed, but would eliminate considerable investments in capital equipment in that under pneumatic start systems, pressurized and/or conditioned air has frequently had to be provided at airline gates or via carts and the like.
Recently, with the emergence of permanent-magnet (samarium-cobalt) generators, it has become possible to consider the operation of these generators as synchronous motor-starters. At least one corporation has conducted studies which show that the use of aircraft synchronous-generators as synchronous motor-starters is a viable alternative. In at least one study, a 150 kva samarium-cobalt generator was used and its operation in the start mode effected through a variable speed constant-frequency (VSCF) static power converter. The power converter in the "generator mode" changes the high variable frequency of the generator into 3 phase 400 Hz 200 VAC constant-frequency power for aircraft use.
In co-pending U.S. patent application Ser. No. 173,111, filed July 28, 1980, entitled "A Direct Driven Aircraft Generating System Providing Variable And Constant Levels Of Electric Power" (assigned to the assignee herein), a unique power generator system is described which makes direct use of the major part of the power developed by the generator. As a consequence, there is no large capacity static power converter in each power channel as there is in the above VSCF type power system. Rather, a separate "dedicated" inverter is provided. As described in another co-pending U.S. patent application, U.S. Ser. No. 183,079 for "Direct-Driven Generator System For Environmental Control System And Engine Starting", filed Sept. 2, 1980, and also assigned to the assignee herein, the separate "dedicated" inverter is used to produce a synthesized form of ac power which is impressed on the starter-generator in such a manner that, as the voltage and frequency of the synthesized ac wave is increased, the high pressure rotor of the engine is brought up to its "self-supporting-speed." In such a circumstance the power for engine starting can be derived from external power, an APU-driven generator or from another engine-driven generator.
One problem with the latter starting technique is that the "inverter" becomes fairly large in electrical capacity, size and weight when it is required to start large turbo-fan engines, such as the Rolls Royce RB 211 engine. Thus, present jet engine starting technology suffers from two basic limitations: (1) pneumatic start systems are fuel inefficient, heavy and require pressurized air, ducts, support apparatus, etc. incompatible with all-electric airplane technology, and (2) advanced technology electric engine start techniques involve high capacity large/heavy inverter/converters. The present invention obviates these problems by providing a new approach which utilizes a "free turbine" APU-generator system to start aircraft jet engines, in which the speed of the free turbine is controlled via "customized" APU controls.
There are several U.S. patents which relate generally to electric engine starting systems and controls and which are of general interest but which do not deal specifically with an APU-generator system and controls of the type taught by the instant invention. Typical of these are U.S. Pat. Nos. 3,753,002 to Jacobson, 3,764,815 to Habock, 3,772,526 to Alwers, 4,069,424 to Burkett, and 4,256,972 to Wyatt.
The '424 patent discloses a system which utilizes an electronic static power supply to power a "parking bus" to which multiple generator-sets may be connected to permit their continued motoring after their prime-movers have been shut down. The '002 patent discloses an ac motor-transfer system to permit large motors to be switched to a line-frequency bus without major perturbations to the system. Multiple motors can be individually-started and connected to the main bus.
Also of general interest are the disclosures in the '815 and '972 patents. In the '815 patent a basic starting system is disclosed which employs a static inverter and a generator used as a starting motor while the '972 patent discloses a system for switching a motor bus to an auxiliary power source upon phase and frequency synchronization so as to provide a "non-interrupted" power changeover.
Finally, the '526 patent relates to a methodology for starting a gas turbine connected to a synchronous generator so that the turbine-generator set may be accelerated up to a point that it can be synchronized and connected to the main power lines. Auxiliary start means for the gas turbine are provided by a diesel engine or electric motor.
None of the aforementioned prior art disclosures (hereby incorporated by reference) are directed to a free turbine APU-driven generator system as disclosed herein for starting aircraft engines. Thus, there remains a need for a highly efficient and lightweight engine start system for use in starting jet engines, particularly for use with future high technology all-electric type aircraft.
From the foregoing, it can be seen then that it is a primary object of this invention to provide a novel aircraft engine starting system which utilizes APU free turbine driven generators.
It is also an object of the present invention to provide an AC motor starting system which utilizes "starting inverters" significantly down-sized from prior art "starting inverters."
A further object of this invention is to provide a turbine APU-generator aircraft engine start system which utilizes a nominally-low external power supply to support engine start, and which in particular can utilize external power cooperatively with the APU.
A still further object of this invention is to provide an aircraft engine starter system which improves engine starting characteristics.